1. Field of the Invention
The present invention relates to a nitride-based semiconductor laser device and a method of forming the same, and more particularly, it relates to a nitride-based semiconductor laser device including an electrode layer and a method of forming the same.
2. Description of the Prior Art
A nitride-based semiconductor laser device is recently expected for application to a light source for a future large capacity optical disk, and actively developed. In order to reduce the operating voltage of the nitride-based semiconductor laser device and improve the reliability thereof, the contact resistance of an electrode must inevitably be reduced. In particular, a nitride-based semiconductor has low p-type carrier concentration, and hence it is difficult to attain an excellent ohmic property (low contact resistance) in relation to a p-side electrode. Therefore, a Pd-based electrode material such as a Pd/Au electrode or a Pd/Pt/Au electrode containing Pd having an excellent ohmic property is recently employed as the p-side electrode.
FIG. 35 is a sectional view showing a first conventional nitride-based semiconductor laser device having a Pd-based electrode. The structure of the first conventional nitride-based semiconductor laser device is now described with reference to FIG. 35. In the first conventional nitride-based semiconductor laser device, an AlGaN low-temperature buffer layer 202 of about 15 nm in thickness is formed on a sapphire substrate 201. An undoped GaN layer 203 of about 3 xcexcm in thickness is formed on the AlGaN low-temperature buffer layer 202. An n-type GaN contact layer 204 is formed on the undoped GaN layer 203 in a thickness of about 5 xcexcm. An n-type AlGaN cladding layer 205 of about 1 xcexcm in thickness, an MQW (multiple quantum well) active layer 206, consisting of InGaN, of about 50 nm in thickness and a p-type AlGaN cladding layer 207 of about 300 nm in thickness having a convex portion are formed on the n-type GaN contact layer 204. A p-type GaN contact layer 208 of about 70 nm in thickness is formed on the convex portion of the p-type AlGaN cladding layer 207.
A p-side electrode 209 consisting of a Pd-based electrode having a three-layer structure formed by stacking a Pd layer of about 10 nm in thickness, an Au layer of about 100 nm in thickness and an Ni layer of about 200 nm in thickness in ascending order is formed on the p-type GaN contact layer 208. An SiO2 film 210 is formed to cover the upper surface of the p-side electrode 209 and regions excluding part of the upper surface of the n-type GaN contact layer 204. A pad electrode 211 is formed to cover the SiO2 film 210 and come into contact with the upper surface of the p-side electrode 209.
The layers from the p-type AlGaN cladding layer 207 to the n-type GaN contact layer 204 are partially removed. An n-side electrode 212 is formed to come into contact with the exposed upper surface of the n-type GaN contact layer 204. A pad electrode 213 is formed to come into contact with the n-side electrode 212.
FIGS. 36 to 40 are sectional views for illustrating a process of fabricating the first conventional nitride-based semiconductor laser device having the Pd-based electrode shown in FIG. 35. FIG. 41 is a sectional view showing the first conventional nitride-based semiconductor laser device shown in FIG. 35 mounted on a submount in a junction-up system from the substrate side. The term xe2x80x9cjunction-up systemxe2x80x9d stands for a system of mounting a nitride-based semiconductor laser device on a submount so that the distance between a substrate and the submount is smaller than that between an active layer and the submount. The fabrication process for the first conventional nitride-based semiconductor laser device having the Pd-based electrode is now described with reference to FIGS. 35 to 41.
First, the AlGaN low-temperature buffer layer 202 is grown on the sapphire substrate 201 by MOCVD (metal organic chemical vapor deposition) under a low-temperature condition of about 600xc2x0 C. in a thickness of about 15 nm, in order to relax lattice mismatching. The undoped GaN layer 203 is formed on the AlGaN low-temperature buffer layer 202 by MOCVD in a thickness of about 3 xcexcm.
Thereafter the n-type GaN contact layer 204 of about 5 xcexcm in thickness, the n-type AlGaN cladding layer 205 of about 1 xcexcm in thickness, the MQW active layer 206, consisting of InGaN, of about 50 nm in thickness, the p-type AlGaN cladding layer 207 of about 300 nm in thickness and the p-type GaN contact layer 208 of about 70 nm in thickness are successively formed on the undoped GaN layer 203 by MOCVD.
Then, the layers from the p-type GaN contact layer 208 to the n-type GaN contact layer 204 are partially removed by anisotropic dry etching, as shown in FIG. 37.
Then, a multilayer film of a Pd layer of about 10 nm in thickness, an Au layer of about 100 nm in thickness and an Ni layer of about 200 nm in thickness stacked in ascending order is formed in a striped shape of about 2 xcexcm in width by a lift off method or the like, thereby forming the p-side electrode 209 consisting of the Pd-based electrode having the three-layer structure of the Pd layer, the Au layer and the Ni layer, as shown in FIG. 38. Thereafter the uppermost Ni layer forming the p-side electrode 209 is employed as an etching mask for etching the p-type GaN contact layer 208 while etching the p-type AlGaN cladding layer 207 by about 150 nm by anisotropic dry etching with CF4 gas. Thus, a ridge portion shown in FIG. 39 is formed.
Then, the SiO2 film 210 is formed on the overall surface by plasma CVD and partially removed from a portion of the n-type GaN contact layer 204, as shown in FIG. 40. The n-side electrode 212 is formed on the portion of the n-type GaN contact layer 204 from which the SiO2 film 210 is removed.
Then, part of the SiO2 film 210 located on the upper surface of the p-side electrode 209 consisting of the Pd-based electrode is removed, followed by formation of the pad electrodes 211 and 213 on the p-side electrode 209 and the n-side electrode 212 respectively, as shown in FIG. 35.
The nitride-based semiconductor laser device shown in FIG. 35 is fixed onto a submount (radiation base) 270 fixed to a stem 271 with a fusible material 260 such as solder, as shown in FIG. 41. In this case, the surface (the back surface of the sapphire substrate 201) of the device opposed to the ridge portion is fused to the submount 270 in the junction-up system.
The first conventional nitride-based semiconductor laser device having the p-side electrode 209 consisting of the Pd-based electrode is formed in the aforementioned manner.
In the aforementioned first conventional nitride-based semiconductor laser device having the p-side electrode 209 consisting of the Pd-based electrode, however, the adhesive force of the p-side electrode 209 consisting of the Pd-based electrode to the p-type GaN contact layer 208 is so weak that the p-side electrode 209 consisting of the Pd-based electrode disadvantageously readily peels off in an intermediate stage of the fabrication process. Therefore, it is difficult to improve the reliability of the device.
In the first conventional nitride-based semiconductor laser device having the p-side electrode 209 consisting of the Pd-based electrode, further, heat or stress disadvantageously deteriorates the contact characteristic of the p-side electrode 209 in the step of forming the pad electrode 211 on the p-side electrode 209 or in an assembling step. In this case, contact resistance is increased to disadvantageously increase the operating voltage.
FIG. 42 is a sectional view showing the structure of a second conventional nitride-based semiconductor laser device 350. Referring to FIG. 42, an n-type GaN contact layer 302 of about 5 xcexcm in thickness is formed on a sapphire substrate 301 in the second conventional nitride-based semiconductor laser device 350. An n-type cladding layer 303, consisting of n-type AlGaN, of about 1 xcexcm in thickness and an active layer 304 of about 0.1 xcexcm in thickness are formed on the n-type GaN contact layer 302. A p-type cladding layer 305, consisting of p-type AlGaInN, having a projection is formed on the active layer 304. A p-type GaN contact layer 306 is formed on the projection of the p-type cladding layer 305. The projection of the p-type cladding layer 305 and the p-type GaN contact layer 306 form a ridge portion of about 0.5 xcexcm in thickness. A p-side electrode 310 of about 0.5 xcexcm in thickness is formed on the upper surface of the ridge portion, to come into contact with the p-type GaN contact layer 306.
The layers from the p-type cladding layer 305 to the n-type GaN contact layer 302 are partially removed. A current blocking layer 307 is formed to cover part of the exposed upper surface of the n-type GaN contact layer 302, the side surfaces of the n-type cladding layer 303, the active layer 304 and the p-type cladding layer 305 and the upper surface of the p-type cladding layer 305 while exposing the upper surface of the p-side electrode 310. A p-side pad electrode 311 of about 0.4 xcexcm in thickness is formed on the current blocking layer 307, to cover the ridge portion and come into contact with the p-side electrode 310 on the upper surface of the ridge portion.
An n-side electrode 312 is formed on the surface of the n-type GaN contact layer 302 exposed due to the partial removal of the layers from the p-type cladding layer 305 to the n-type GaN contact layer 302. An n-side pad electrode 313 is formed on the n-side electrode 312.
In the second conventional nitride-based semiconductor laser device 350 having the aforementioned structure, a current flows from the p-side pad electrode 311 to the active layer 304, the n-type cladding layer 303, the n-type GaN contact layer 302, the n-side electrode 312 and the n-side pad electrode 313 through the p-side electrode 310 as well as the p-type GaN contact layer 306 and the p-type cladding layer 305 forming the ridge portion. Thus, the nitride-based semiconductor laser device 350 can generate a laser beam in a region of the active layer 304 located under the ridge portion.
When employed as a light source for a rewritable optical disk, the second conventional nitride-based semiconductor laser device 350 having the aforementioned structure must perform a high-output operation with an optical output of about or at least 30 mW. When the nitride-based semiconductor laser device 350 performs a high-output operation, however, the heating value is generally increased to disadvantageously deteriorate the life of the nitride-based semiconductor laser device 350.
In order to radiate heat caused by the high-output operation of the nitride-based semiconductor laser device 350, therefore, a method of assembling the laser device 350 in close contact with a submount (radiation base) or a stem is employed so that the distance between the active layer 304 and the submount or the stem is smaller than that between the sapphire substrate 301 and the submount or the stem. This fixing method is referred to as a junction-down method. In particular, the nitride-based semiconductor laser device 350 having a higher operating voltage than an AlGaAs-based infrared semiconductor laser device or an AlGaInP-based red semiconductor laser device exhibits a high heating value. In order to operate the nitride-based semiconductor laser device 350 with a high output, therefore, junction-down assembly excellent in heat radiation is necessary.
FIG. 43 is a schematic diagram showing the second conventional nitride-based semiconductor laser device 350 in a state assembled in the junction-down system. Referring to FIG. 43, the second conventional nitride-based semiconductor laser device 350 is fixed to a submount 370 with a fusible material 360. The submount 370 is fixed to a stem 371.
When the aforementioned second conventional nitride-based semiconductor laser device 350 is assembled in the junction-down system, however, the active layer (emission part) 304 of the nitride-based semiconductor laser device 350 is disadvantageously covered with the fusible material 360 due to the small distance between the active layer 304 and the fusible material 360. This problem is now described in detail.
An infrared laser or a red laser can strongly confine light in an active layer since the difference between the refractive indices of the active layer and a cladding layer can be increased. Therefore, the distance between the active layer (emission part) and a fusible material can be increased by providing a contact layer having a large thickness. Thus, the aforementioned problem of the active layer (emission part) covered with the fusible material can be solved.
In the nitride-based semiconductor laser device 350, however, the difference between the refractive indices of the p-type GaN contact layer 306 employed as a contact layer and the p-type cladding layer 305 and that of the active layer 304 is not so large. When increased in thickness, therefore, the p-type GaN contact layer 306 functions as a light guide layer to readily generate a higher mode. This leads to such a new problem that it is difficult to mode-control the nitride-based semiconductor laser device 350. Therefore, it is difficult to increase the thickness of the p-type GaN contact layer 306 in the second conventional nitride-based semiconductor laser device 350. Consequently, it is generally difficult to solve the problem of the active layer (emission part) 304 of the nitride-based semiconductor laser device 350 covered with the fusible material 360.
FIGS. 44 to 46 are sectional views for illustrating problems caused in the second conventional nitride-based wee, semiconductor laser device 350 assembled in the junction-down system. In order to mount the second conventional nitride-based semiconductor laser device 350 on the submount 370, the upper surface of the p-side pad electrode 311 is pressed against and fused to the submount 370 with the fusible material 360 such as solder under heat and a pressure. At this time, the fusible material 360 partially creeps up along the front end surface of the nitride-based semiconductor laser device 350 closer to the ridge portion as shown in FIG. 44, due to the small distance between the active layer 304 and the upper surface of the p-side pad electrode 311. In general, therefore, the active layer 304 serving as an emission part is disadvantageously covered with the fusible material 360. In this case, the emission characteristic of the nitride-based semiconductor laser device 350 is disadvantageously deteriorated.
As shown in FIG. 45, part of the fusible material 360 further creeps up along the front end surface of the nitride-based semiconductor laser device 350 closer to the ridge portion up to p-n junction parts located on the upper and lower surfaces of the active layer 304 serving as the emission part, to disadvantageously cause shorting. When shorted, the device 350 is disadvantageously rendered inoperable. As shown in FIG. 46, further, part of the fusible material 360 may creep up along the side surface of the nitride-based semiconductor laser device 350 up to the n-type cladding layer (p-n junction part) 303 beyond the active layer 304, to cause shorting.
When the nitride-based semiconductor laser device 350 is deteriorated in emission characteristic or defectively shorted as shown in FIGS. 44 to 46, the yield of junction-down assembly is disadvantageously reduced.
In general, further, the distance between the ridge portion and the upper surface of the p-side pad electrode 311 is so small that the heat and the pressure for fusing the fusible material 360 are readily transmitted to the ridge portion, as shown in FIGS. 44 to 46. Therefore, the operating voltage of the nitride-based semiconductor laser device 350 is increased due to the heat and the pressure. Consequently, the heating value is increased in operation. Therefore, the life of the nitride-based semiconductor laser device 350 is disadvantageously reduced.
Also when the second conventional nitride-based semiconductor laser device 350 is directly fixed to a stem (not shown), the active layer 304 or the n-type cladding layer 303 located on the active layer 304 is disadvantageously covered with a fusible material for fixing the nitride-based semiconductor laser device 350 to the stem. Thus, the nitride-based semiconductor laser device 350 is deteriorated in emission characteristic or defectively shorted also in this case.
In general, a method of reducing the thickness of the fusible material 360 deposited to the submount 370 so that the active layer 304 is not covered with the fusible material 360 or a method of reducing the amount of the pellet-type fusible material 360 when directly fixing the nitride-based semiconductor laser device 350 to the stem is proposed. According to this method, however, the nitride-based semiconductor laser device 350 cannot be reliably fused to the submount 370, leading to such another problem that the nitride-based semiconductor laser device 350 peels off from the submount 370. Also in this case, therefore, the yield of junction-down assembly is disadvantageously reduced.
The present invention has been proposed in order to solve the aforementioned problems.
An object of the present invention is to provide a nitride-based semiconductor laser device having a low operating voltage and high reliability.
Another object of the present invention is to increase the adhesive force of the overall electrode layer to a nitride-based semiconductor layer without damaging a low contact property in the aforementioned nitride-based semiconductor laser device.
Still another object of the present invention is to provide a method of forming a nitride-based semiconductor laser device capable of readily forming a nitride-based semiconductor laser device having a low operating voltage and high reliability.
A further object of the present invention is to provide a nitride-based semiconductor laser apparatus capable of improving the yield of junction-down assembly.
A further object of the present invention is to provide a nitride-based semiconductor laser apparatus capable of preventing reduction of a device life.
A further object of the present invention is to provide a nitride-based semiconductor laser apparatus capable of preventing deterioration of a light emission characteristic in junction-down assembly.
A further object of the present invention is to provide a nitride-based semiconductor laser apparatus capable of preventing shorting in junction-down assembly.
A nitride-based semiconductor laser device according to a first aspect of the present invention comprises a nitride-based semiconductor layer formed on an active layer and an electrode layer formed on the nitride-based semiconductor layer, while the electrode layer includes a first electrode layer containing a material having strong adhesive force to the nitride-based semiconductor layer and a second electrode layer, formed on the first electrode layer, having weaker adhesive force to the nitride-based semiconductor layer than the first electrode layer for reducing contact resistance of the electrode layer with respect to the nitride-based semiconductor layer.
In the nitride-based semiconductor laser device according to the first aspect, the first electrode layer containing the material having strong adhesive force to the nitride-based semiconductor layer is provided on the nitride-based semiconductor layer while the second electrode layer reducing the contact resistance of the electrode layer with respect to the nitride-based semiconductor layer is provided on the first electrode layer as hereinabove described, whereby the adhesive force of the overall electrode layer to the nitride-based semiconductor layer can be increased due to the first electrode layer and low contact resistance can be attained due to the second electrode layer. Thus, the device can be improved in reliability and reduced in operating voltage.
In the nitride-based semiconductor laser device according to the aforementioned first aspect, the second electrode layer preferably has lower contact resistance with respect to the nitride-based semiconductor layer than the first electrode layer. According to this structure, low contact resistance can be readily attained due to the second electrode layer. The first electrode layer preferably has a thickness of not more than 3 nm. When the first electrode layer is formed with such a small thickness of not more than 3 nm, the adhesive force of the overall electrode layer to the nitride-based semiconductor layer can be increased due to the first electrode layer without deteriorating the low contact property of the second electrode layer.
In the nitride-based semiconductor laser device according to the aforementioned first aspect, the first electrode layer preferably contains at least one material selected from a group consisting of Pt, Ni, Cr, Ti, Hf and Zr, and the second electrode layer preferably contains Pd. According to this structure, the adhesive force of the overall electrode layer to the nitride-based semiconductor layer can be readily increased due to the first electrode layer while low contact resistance can be readily attained due to the second electrode layer. In this case, the first electrode layer more preferably includes a Pt layer, and the second electrode layer more preferably includes a multilayer film having a Pd layer. In this case, the uppermost layer of the second electrode layer preferably includes a metal layer serving as an etching mask. According to this structure, the uppermost layer of the second electrode layer can be employed as an etching mask for forming a ridge portion, whereby no additional etching mask may be formed. Consequently, the fabrication process can be simplified.
The aforementioned nitride-based semiconductor laser device preferably further comprises a mixed layer, formed between the first and second electrode layers, including a Pt layer and a Pd layer. According to this structure, the Pd layer is formed closer to the nitride semiconductor layer, so that the contact resistance can be reliably reduced with Pd.
In the aforementioned nitride-based semiconductor laser device, the nitride-based semiconductor layer preferably has an irregular surface. According to this structure, the contact area between the nitride-based semiconductor layer and the first electrode layer can be increased, whereby the contact resistance can be further reduced. In this case, the nitride-based semiconductor layer having the irregular surface has an In composition of at least 3% and a thickness of not more than 20 nm. When the nitride-based semiconductor layer is formed with such a composition in such a thickness, the surface of the nitride-based semiconductor layer can be readily irregularized.
In the aforementioned nitride-based semiconductor laser device, the nitride-based semiconductor layer preferably includes a contact layer formed on a convex portion of a cladding layer, and the convex portion of the cladding layer and the contact layer form a ridge portion. According to this structure, the electrode layer must be formed on the contact layer having a narrow area. Also in this case, the adhesive force of the overall electrode layer to the contact layer forming the ridge portion can be increased due to the first electrode layer while low contact resistance can be attained due to the second electrode layer, whereby the device can be improved in reliability and reduced in operating current and operating voltage.
The aforementioned nitride-based semiconductor laser device preferably further comprises a base for mounting an element including the nitride-based semiconductor layer, the first electrode layer and the second electrode layer from the side of the active layer. According to this structure, heat generated from the active layer in emission can be excellently radiated through the base. In such junction-down assembly, the electrode layer tends to peel due to stress readily applied to the ridge portion. According to the present invention, the adhesive force of the overall electrode layer to the nitride-based semiconductor layer can be increased due to the first electrode layer, whereby the electrode layer can be effectively prevented from peeling also in junction-down assembly.
A nitride-based semiconductor laser device according to a second aspect of the present invention comprises a nitride-based semiconductor layer formed on an active layer and an electrode layer formed on the nitride-based semiconductor layer, while the electrode layer includes a first electrode layer containing a material having strong adhesive force to the nitride-based semiconductor layer and a second electrode layer, formed on the first electrode layer, having weaker adhesive force to the nitride-based semiconductor layer than the first electrode layer for reducing an energy barrier of the electrode layer against the nitride-based semiconductor layer.
In the nitride-based semiconductor laser device according to the second aspect, the first electrode layer containing the material having strong adhesive force to the nitride-based semiconductor layer is provided on the nitride-based semiconductor layer and the second electrode layer reducing the energy barrier of the electrode layer against the nitride-based semiconductor layer is provided on the first electrode layer as hereinabove described, whereby the adhesive force of the overall electrode layer to the nitride-based semiconductor layer can be increased due to the first electrode layer and low contact resistance can be attained due to the second electrode layer. Therefore, the device can be improved in reliability and reduced in operating voltage.
A method of forming a nitride-based semiconductor laser device according to a third aspect of the present invention comprises steps of forming a nitride-based semiconductor layer on an active layer and forming an electrode layer on the surface of the nitride-based semiconductor layer, while the step of forming the electrode layer includes steps of forming a first electrode layer containing a material having strong adhesive force to the nitride-based semiconductor layer and forming a second electrode layer having weaker adhesive force to the nitride-based semiconductor layer than the first electrode layer for reducing contact resistance of the electrode layer with respect to the nitride-based semiconductor layer on the first electrode layer.
In the method of forming a nitride-based semiconductor laser device according to the third aspect, the first electrode layer containing the material having strong adhesive force to the nitride-based semiconductor layer is formed on the surface of the nitride-based semiconductor layer and the second electrode layer reducing the contact resistance of the electrode layer with respect to the nitride-based semiconductor layer is formed on the first electrode layer as hereinabove described, whereby the adhesive force of the overall electrode layer to the nitride-based semiconductor layer can be increased due to the first electrode layer and low contact resistance can be attained due to the second electrode layer. Thus, the device can be improved in reliability, and a nitride-based semiconductor laser device reducible in operating voltage can be readily formed.
In the method of forming a nitride-based semiconductor laser device according to the aforementioned third aspect, the second electrode layer preferably has lower contact resistance with respect to the nitride-based semiconductor layer than the first electrode layer. According to this structure, low contact resistance can be readily attained due to the second electrode layer. The first electrode layer preferably has a thickness of not more than 3 nm. When the first electrode layer is formed with such a small thickness of not more than 3 nm, the adhesive force of the overall electrode layer to the nitride-based semiconductor layer can be increased due to the first electrode layer without deteriorating the low contact property of the second electrode layer.
In the method of forming a nitride-based semiconductor laser device according to the third aspect, the step of forming the first electrode layer on the surface of the nitride-based semiconductor layer preferably includes a step of forming the first electrode layer and the second electrode layer on the surface of the nitride-based semiconductor layer and thereafter feeding a current between the second electrode layer and the nitride-based semiconductor layer thereby partially moving material included in the second electrode layer to a portion close to the surface of the nitride-based semiconductor layer. According to this structure, the second electrode layer can further exhibit the low-contact property without reducing the adhesive force to the nitride-based semiconductor layer improved by the first electrode layer.
In the aforementioned method of forming a nitride-based semiconductor laser device, the step of forming the first electrode layer preferably includes a step of forming the first electrode layer by any of electron beam heating evaporation, resistance heating evaporation and sputtering evaporation. When such evaporation is employed, the first electrode layer containing the material having strong adhesive force to the nitride-based semiconductor layer can be readily formed.
In the aforementioned method of forming a nitride-based semiconductor laser device, the nitride-based semiconductor layer preferably includes a contact layer formed on a cladding layer, the step of forming the second electrode layer preferably includes a step of forming the second electrode layer on a prescribed region of the upper surface of the first electrode layer by a lift off method, and the method preferably further comprises a step of forming a ridge portion by partially etching the first electrode layer, the contact layer and the cladding layer through the second electrode layer serving as a mask after forming the second electrode layer. According to this structure, the ridge portion consisting of a convex portion of the cladding layer and the contact layer can be readily formed. When the first electrode layer is patterned not by the lift off method but by etching, pattern peeling readily caused in the lift off method can be prevented.
In the aforementioned method of forming a nitride-based semiconductor laser device, the first electrode layer preferably contains at least one material selected from a group consisting of Pt, Ni, Cr, Ti, Hf and Zr, and the second electrode layer preferably contains Pd. According to this structure, the adhesive force of the overall electrode layer to the nitride-based semiconductor layer can be readily increased due to the first electrode layer and low contact resistance can be readily attained due to the second electrode layer.
In the aforementioned method of forming a nitride-based semiconductor laser device, the step of forming the nitride-based semiconductor layer preferably includes a step of forming the nitride-based semiconductor layer having an irregular surface. According to this structure, the contact area between the nitride-based semiconductor layer and the first electrode layer can be increased, whereby the contact resistance can be further reduced. In this case, the nitride-based semiconductor layer having the irregular surface preferably has an In composition of at least 3% and a thickness of not more than 20 nm. When the nitride-based semiconductor layer is formed with such a composition in such a thickness, the surface of the nitride-based semiconductor layer can be readily irregularized.
A nitride-based semiconductor laser apparatus according to a fourth aspect of the present invention comprises a nitride-based semiconductor layer having an active layer and a ridge potion formed on the active layer and a first electrode layer, formed to come into contact with an exposed upper surface of the ridge portion, having a thickness larger than the distance between the lower surface of a cladding layer located under the active layer and the upper surface of the ridge portion.
The nitride-based semiconductor laser apparatus according to the fourth aspect is provided with the first electrode layer having the thickness larger than the distance between the lower surface of the cladding layer located under the active layer and the upper surface of the ridge portion, thereby increasing the distance between the active layer and the upper surface of the first electrode layer. When the upper surface of the first electrode layer is fixed to a base for heat radiation with a fusible material, therefore, the distance between the fusible material and the active layer is so increased that the active layer (emission part) can be prevented from being covered with the fusible material. Consequently, the nitride-based semiconductor laser apparatus can be prevented from deterioration of the emission characteristic. The distance between the fusible material and the active layer is increased, whereby p-n junction parts located on the upper and lower surfaces of the active layer (emission part) can be prevented from being covered with the fusible material. Consequently, the nitride-based semiconductor laser apparatus can be prevented from shorting. The nitride-based semiconductor laser apparatus can be prevented from deterioration of the emission characteristic and shorting as described above, whereby the assembly yield can be improved. The distance between the fusible material and the active layer is increased, whereby the thickness of the fusible material layer or the quantity of the fusible material can be increased within the range not covering the active layer (emission part). Thus, a laser device including the nitride-based semiconductor layer and the first electrode layer can be reliably fused to the base for heat radiation, whereby the laser device can be effectively prevented from peeling off from the base. Thus, the assembly yield can be improved.
The distance between the ridge portion and the upper surface of the first electrode layer is also increased, whereby heat is hardly transmitted to the ridge portion when the first electrode layer is fused with the fusible material, while a pressure transmitted to the ridge portion can be absorbed due to the increased thickness of the first electrode layer consisting of a relatively soft material. Thus, the operating voltage of the nitride-based semiconductor laser apparatus can be prevented from being increased by heat and the pressure in fusion, whereby increase of the heating value can be prevented. Consequently, the nitride-based semiconductor laser apparatus can be prevented from reduction of the device life.
In the nitride-based semiconductor laser apparatus according to the aforementioned fourth aspect, the first electrode layer may include a first metal layer coming into contact with the ridge portion and a second metal layer formed on the first metal layer so that its surface is exposed, and the second metal layer may have a larger thickness than the first metal layer. According to this structure, the first metal layer and the second metal layer having a larger thickness than the first metal layer can readily increase the thickness of the first electrode layer. Thus, the distance between the fusible material and the active layer can be increased when fixing the upper surface of the first electrode layer to the base for heat radiation with the fusible material. In this case, the second metal layer preferably includes a p-side pad electrode consisting of Au. According to this structure, a pressure transmitted to the ridge potion can be absorbed due to the increased thickness of the p-side pad electrode consisting of Au, which is a relatively soft material. Thus, increase of the operating voltage resulting from the pressure in fusion can be prevented, whereby increase of the heating value can also be prevented. Consequently, the nitride-based semiconductor laser apparatus can be prevented from reduction of the device life.
The aforementioned nitride-based semiconductor laser apparatus further comprises a current blocking layer formed to cover regions excluding the upper surface of the ridge portion, and the first metal layer is formed not only on the ridge portion but also on the current blocking layer. According to this structure, the contact area between the first and second metal layers is increased, whereby heat can be excellently radiated from the first metal layer to the second metal layer. In this case, the nitride-based semiconductor laser apparatus preferably further comprises a contact layer, consisting of a nitride-based semiconductor, formed on the ridge portion and the current blocking layer, and the first metal layer is preferably formed on the ridge portion and the current blocking layer through the contact layer. According to this structure, the contact area between the contact layer and the first metal layer is increased, whereby heat can be excellently radiated from the contact layer to the first metal layer. In the aforementioned nitride-based semiconductor laser apparatus, the first metal layer may include a multilayer film consisting of different metals.
In the nitride-based semiconductor laser apparatus according to the aforementioned fourth aspect, the first electrode layer may include a first metal layer coming into contact with the ridge portion and a second metal layer formed on the first metal layer so that its surface is exposed, and the first metal layer may have a larger thickness than the second metal layer. According to this structure, the second metal layer and the first metal layer having a larger thickness than the second metal layer can readily increase the thickness of the first electrode layer. Thus, the distance between the fusible material and the active layer can be increased when fixing the upper surface of the first electrode layer to the base for heat radiation with the fusible material. In this case, the first metal layer preferably includes a p-side electrode having an Au film larger in thickness than the second metal layer. According to this structure, a pressure transmitted to the ridge portion can be absorbed due to the increased thickness of the p-side electrode consisting of Au, which is a relatively soft material. Thus, increase of the operating voltage resulting from the pressure in fusion can be prevented, whereby increase of the heating value can also be increased. Consequently, the nitride-based semiconductor laser apparatus can be prevented from reduction of the device life. In this case, the first metal film may include a multilayer film consisting of different metals.
In the aforementioned nitride-based semiconductor laser apparatus, the first metal layer having a larger thickness than the second metal layer is preferably formed to project only on the upper surface of the ridge portion. According to this structure, the step between a projection on the ridge portion and the remaining potions is so increased that the ridge portion and an emission point located immediately under the ridge portion can be readily distinguished from each other. Consequently, the position of the emission point can be precisely controlled in junction-down assembly.
In the nitride-based semiconductor laser apparatus according to the aforementioned fourth aspect, the first electrode layer preferably includes a first metal layer coming into contact with the ridge portion, a second metal layer formed on the first metal layer and a third metal layer formed on the second metal layer, and the third metal layer preferably has a larger thickness than the first metal layer and the second metal layer. According to this structure, the first and second metal layers and the third metal layer having a larger thickness than the first and second metal layers can readily increase the thickness of the first electrode layer. Thus, the distance between the fusible material and the active layer can be increased when fixing the upper surface of the first electrode layer to the base for heat radiation with the fusible material.
In this case, the third metal layer preferably includes a p-side thick-film electrode having an Au film larger in thickness than the first metal layer and the second metal layer. According to this structure, the pressure transmitted to the ridge portion can be absorbed due to the increased thickness of the p-side thick-film electrode consisting of Au, which is a relatively soft material. Thus, the operating voltage can be prevented from being increased due to the pressure in fusion, whereby increase of the heating value can also be prevented. Consequently, the nitride-based semiconductor laser apparatus can be prevented from reduction of the device life.
In the aforementioned nitride-based semiconductor laser apparatus, each of the first metal layer, the second metal layer and the third metal layer may have a multilayer structure.
The nitride-based semiconductor laser apparatus according to the aforementioned fourth aspect preferably further comprises a current blocking layer formed to cover regions excluding the upper surface of the ridge portion, and the current blocking layer preferably includes either a nitride-based semiconductor having a conductivity type different from that of the ridge portion or an insulator film. In this case, the current blocking layer may include a current blocking layer consisting of an SiO2 film, or a current blocking layer consisting of any material of AlGaN, InGaN and GaN having a conductivity type different from that of the ridge portion.
In the nitride-based semiconductor laser apparatus comprising the aforementioned current blocking layer, the current blocking layer may consist of a nitride-based semiconductor having a conductivity type different from that of the ridge portion, and the nitride-based semiconductor laser apparatus may further comprise a second electrode layer formed on a surface exposed by partially removing the nitride-based semiconductor layer and a protective film consisting of an insulator film formed on a side surface exposed by partially removing the nitride-based semiconductor layer. In this case, the current blocking layer may include a current blocking layer consisting of AlGaN, and the protective film may include a protective film consisting of SiO2.
The nitride-based semiconductor laser apparatus according to the aforementioned fourth aspect preferably further comprises a base for mounting an element including the nitride-based semiconductor layer and the first electrode layer from the side of the active layer. According to this structure, heat from the active layer can be excellently radiated through the base in emission. In this case, the base may include a submount.
A nitride-based semiconductor laser apparatus according to a fifth aspect of the present invention comprises a nitride-based semiconductor layer having an active layer and a ridge portion formed on the active layer and a first electrode layer, formed to come into contact with an exposed upper surface of the ridge portion, having a thickness of at least 2 xcexcm.
The nitride-based semiconductor laser apparatus according to the fifth aspect is provided with the first electrode layer having the large thickness of at least 2 xcexcm coming into contact with the exposed upper surface of the ridge portion, thereby increasing the distance between the upper surface of the first electrode layer and the upper surface of the ridge portion. Thus, the distance between a fusible material and the active layer is increased when fixing the upper surface of the first electrode layer to a base for heat radiation with the fusible material, whereby the active layer (emission part) can be prevented from being covered with the fusible material. Consequently, the nitride-based semiconductor laser apparatus can be prevented from deterioration of the emission characteristic.
The distance between the fusible material and the active layer is increased, whereby p-n junction parts located on the upper and lower surfaces of the active layer (emission part) can be prevented from being covered with the fusible material. Consequently, the nitride-based semiconductor laser apparatus can be prevented from shorting. The nitride-based semiconductor laser apparatus can be prevented from deterioration of the emission characteristic and shorting as described above, whereby the assembly yield can be improved. The distance between the fusible material and the active layer is increased, whereby the thickness of the fusible material layer or the quantity of the fusible material can be increased within the range not covering the active layer (emission part). Thus, a laser device including the nitride-based semiconductor layer and the first electrode layer can be reliably fused to the base for heat radiation, whereby the laser device can be effectively prevented from peeling off from the base. Thus, the assembly yield can be improved.
The distance between the ridge portion and the upper surface of the first electrode layer is also increased, whereby heat is hardly transmitted to the ridge portion when the first electrode layer is fused with the fusible material, while a pressure transmitted to the ridge portion can be absorbed due to the increased thickness of the first electrode layer consisting of a relatively soft material. Thus, the operating voltage of the nitride-based semiconductor laser apparatus can be prevented from being increased by heat and the pressure in fusion, whereby increase of the heating value can be prevented. Consequently, the nitride-based semiconductor laser apparatus can be prevented from reduction of the device life.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.